Space based planetary imagers are useful for remote sensing of atmospheric compositions, crop assessments, weather prediction and other types of monitoring activities. Monochromatic and multispectral satellite-based, remote sensors are able to measure properties of the atmosphere above the earth, when their detector arrays are properly calibrated for radiometric response.
A method of calibrating the radiance measured by these remote sensors is to create a reference radiation using a known source of spectral radiance, such as the sun. The radiation from the sun may be used as a reference signal into a diffusive reflector which, in turn, may provide a known radiance to a remote sensor for calibrating its one or more detector arrays.
The output of a detector array may be measured as the remote sensor receives the known reflected energy from the diffusive reflector. This radiance calibration method provides sufficient information to correctly measure and calculate other types of radiance incident on the remote sensor during normal operation, such as radiance from views of the Earth or other targets of interest. The spectral characteristics of a diffusive reflector, or diffuser panel, however, may change with time due to degradation of the diffuser panel. Since the diffuser panel is employed as the reference source, any change, i.e., degradation in the diffusive surface material, results in distortion in the measurements of the remote sensor.
Techniques have been suggested for calibrating Earth viewing spectrometers. One technique suggests viewing the sun directly to calibrate an Earth viewing spectrometer. Viewing the sun directly, however, requires using precision attenuators. Such attenuators may include a 500 micron pinhole, a 0.1 throughput neutral density filter and a CCD array, in which the latter is sub-sampled in time by a factor of 10. Using these attenuators results in the necessary attenuation of 1/100,000, and allows the sun to be used as a calibration source.
The aforementioned solar attenuation technique has been proposed for calibrating an Earth viewing solar wavelength spectrometer, such as the Climate Absolute Radiance and Refractivity Observatory (CLARREO), which is intended for monitoring the albedo from the Earth over a long time duration. In order to accomplish its mission, CLARREO must have an absolute accuracy of 0.3% and a calibration stability of 0.1% per decade.
Proposing the solar attenuation technique to calibrate CLARREO, however, raises several concerns. These concerns include lack of precise knowledge and stability of the proposed 500 micron pinhole. There is also concern about the performance of the primary mirror in CLARREO for viewing the Earth. This concern is due to potential burn in the mirror, or degradation of spots on the mirror, thereby causing changes in the response characteristics of the mirror. Furthermore, although throughput of the proposed neutral density filter is to be measured regularly by viewing the moon (i.e. with the filter in and out of the optical train), nevertheless, the lunar albedo is currently known to a low accuracy value of approximately 5-10%. This value may also change by several percent in a matter of minutes as the moon orbits the Earth. Finally, the time sub-sampling of the CCD array raises concerns over its accuracy and linearity.
The U.S. Geological Service (USGS) has a Robotic Lunar Observatory (ROLO), which has made daily measurements of the solar spectral irradiance scattered from the moon in the Arizona desert. These measurements allowed construction of a database model of ROLO that provides a spectrally resolved albedo of the moon for all phase/libration angles.
As it is based on measurements made from Earth, the ROLO model has to account for the absorption of the atmosphere above Arizona. For this reason, the ROLO model has excellent stability over time, but its absolute accuracy is estimated to be at a 5-10% level, which is not sufficient to meet the CLARREO goal of 0.3% absolute accuracy.
The present invention addresses this problem, and provides a method and system for calibrating a climate monitoring remote sensor, such as CLARREO, which requires a spectrometer with a 0.3% absolute accuracy and a 0.1% per decade calibration stability.
As will be explained, the present invention provides a raster scan integral method to raster scan the moon using spectrometers of existing technology. These spectrometers are calibrated by the present invention and then used for determining the albedo of the Earth, at an initial 5-10% accuracy and a 0.1% per decade calibration stability. The 5-10% accuracy is too low to satisfy requirements, however, because the calibration also relies on the database model of ROLO, which has the low accuracy of 5-10%.
Later in time, similar spectrometers may be installed on the International Space Station (ISS), along with a standardized, traceable measurement bench. These later spectrometers also use the raster scan integral of the present invention to separately raster scan the moon and the sun. Using the measurement bench, these later spectrometers may be calibrated to an accuracy of 0.3%, which satisfies requirements.
Furthermore, the present invention allows for the updating of the previous, low accuracy albedo data of the Earth that have been obtained from the earlier spectrometers, which relied on the database model of ROLO for their calibration. Once updated, the albedo data of the Earth obtained from the earlier spectrometers will be corrected to the required absolute accuracy. Thus, the present invention is effective in obtaining the albedo of the Earth over a long time duration, at the required 0.3% accuracy and 0.1% per decade stability.